This invention relates to apparatus for feeding particulate material, such as coal particles, to be burned in a coal combustion system in an electrical power plant in which the coal is fed at a controlled rate to a furnace or other component. More particularly, the invention relates to a gravimetric feeder whose rate of feed is monitored and automatically controlled, and especially to a device for calibrating the weight-sensing and feed rate control system.
Although the present invention is particularly adapted for feeding coal in electrical power plants, the invention is useful in the feeding of particulate material in general and it is not intended that the basic concept of the invention be restricted to feeding coal.
In many coal-fired electrical power plants, coal is fed downwardly by gravity from overhead hoppers or bunkers to pulverizers or other power plant components such as boiler stokers or cyclone burners. The feed rate may be controlled by a gravimetric feeder such as that shown and described in U.S. Pat. No. 3,187,944, the disclosure of which is incorporated by reference herein and made a part hereof.
Generally, the gravimetric feeder receives the particulate coal in the form of a continuous stream in a fixed flow path, and delivers the coal at a controlled rate as free-falling or separately flowing particles. The rate of discharge by weight is controllable to achieve accurate control of the fuel-air ratio. A feeder of this type enables the coal flow to be divided equally among several firing units of a single boiler.
Such feeders utilize a horizontal conveyor located within a sealed enclosure and adapted to contain gas at a pressure higher than ambient atmospheric air pressure. The conveyor includes an endless belt with an upper span or run that receives coal through an inlet in the enclosure in the form of a column from the hopper. The upper belt run transports a layer of the coal at a level controlled by an adjustable leveling bar or valve across a weighing span of predetermined length, which weighing span is supported by one or more rollers, one roller being a weighing roller. The weighing roller thus carries a predetermined portion of the weight of coal on the weighing span and the weight carried thereby is sensed by a weight-sensing mechanism including, for example, load cells of the type readily obtainable from commercial sources. If the weight sensed varies from that desired, then the weighing mechanism automatically moves the leveling bar to adjust the depth (and thus the weight) of coal transported on the belt.
It is particularly important in order to assure accurate feed rate control that the weighing mechanism be accurately calibrated. Over a period of time, the mechanism tends to change its condition of adjustment due to vibration, wear, environmental conditions, etc. Accordingly, it is desirable that the equipment have provision for applying a test load to the mechanism at frequent intervals with a minimum of effort in order to check calibration and make any necessary adjustments.
The apparatus of the present invention satisfies the requirements indicated and affords other features and advantages heretofore not obtainable.